How can I get an UpdatePanel to intercept a CompositeControl's DropDownList

Question

I have a CompositeControl that contains a DropDownList.

I have set the AutoPostBack property of the DropDownList to true.

On the page, I have:

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Answer 1

I made the following at the end of the BuildControlHierarchy:

        ...
        if (DesignMode || Page == null) return;

        var sm = ScriptManager.GetCurrent(Page);
        if (sm == null)
        {
            throw new MissingFieldException("The ScriptManager is needed on the page!");
        }
        sm.RegisterAsyncPostBackControl();